1. Field of the Invention
The present invention relates generally to television receivers and more particularly is directed to a double-scanning non-interlace television receiver which receives a video signal of an interlace scanning system and displays it with a non-interlace scanning system.
2. Description of the Prior Art
Generally, in the picture display of an interlace scanning system, when one frame contains 525 scanning lines, 262.5 scanning lines constitute one field in which when the vertical frequency is selected as 60 Hz, a field flicker can be suppressed. Also, in order to obtain a picture of high vertical resolution, during the succeeding field the scanning is carried out after being displaced in spacing by one half of the scanning line.
However, in this case, although the number of picture images is given as 60 pictures/sec from a macroscopic point of view, one scanning line at the same position is lit at every 1/30 seconds and the display period is 1/30 seconds from a microscopic point of view. As a result, the light emission of one scanning line is flickered, which then causes a so-called flicker visually. Namely, a line flicker occurs.
In order to reduce the line flicker, it is enough that the display period of one scanning line is selected to be shorter than 1/30 seconds. Therefore, in the art a double-scanning television receiver has been proposed in which the double-speed scanning with the horizontal frequency doubled is performed. In this case, the display periods of both the field and line are selected to be 1/60 seconds so that the field flicker and the line flicker will never occur.
In order to perform the double-speed scanning in which the horizontal frequency is doubled, the video signal of an interlace system is converted into the form of a video signal of an non-interlace system having the horizontal frequency which is doubled and then fed to a picture tube.
As the above converting method, in the art there have been previously proposed methods which are embodied by practical circuits shown in FIGS. 1 to 3, respectively.
First, FIG. 1 shows a method of predicting that a video signal on a preceding scanning line is the same as that on the succeeding scanning line and employing directly the video signal on the preceding scanning line as an interpolated signal, namely, a so-called pre-value interpolation method.
In FIG. 1, reference numeral 1 designates an input terminal to which a video signal S.sub.i of an interlace system is supplied. Reference numerals 2 and 3 respectively designate 1 H (one horizontal period) memories which are each so formed that the read speed is selected to be twice as high as the write speed. Reference numerals 4 and 5 respectively designate change-over switches, each of which is changed in position at each 1 H period so that when the change-over switch 4 is changed in position to one side of the 1 H memories 2 and 3, the change-over switch 5 is changed in position to the other side thereof.
In the example shown in FIG. 1, the video signal S.sub.i applied to the input terminal 1 is alternately written in the 1 H memories 2 and 3 by 1 H period amount each, and in the 1 H period during which the video signal S.sub.i is being written into one of the 1 H memories 2 and 3, the video signal S.sub.i of 1 H period amount written therein during the preceding 1 H period is continuously read out twice from the other one of the 1 H memories 2 and 3 and then delivered to an output terminal 6. Accordingly, in this case, at the output terminal 6 is obtained a video signal S.sub.NI (shown in FIG. 4B) of non-interlace system which has the horizontal frequency twice that of the video signal S.sub.i or in which the video signal of each scanning line of the video signal S.sub.i continuously appears twice each with every 1/2 H periods.
FIG. 2 shows a method of predicting that a video signal on the scanning line to be interpolated is equal to a video signal on a scanning line of a preceding field and employing this video signal as the interpolated signal, namely, a so-called pre-field interpolation method.
In FIG. 2, reference numerals 2A and 3A respectively designate 1 H memories and reference numerals 4A and 5A change-over switches which operate in the same way as the 1 H memories 2 and 3 and the change-over switches 4 and 5 shown in FIG. 1. Also, reference numerals 2B and 3B respectively designate 1 H memories and reference numerals 4B and 5B change-over switches which also operate in the same way as the 1 H memories 2 and 3 and the change-over switches 4 and 5 shown in FIG. 1.
Also in FIG. 2, reference numeral 7 designates a delay line which has a delay amount of 1 V (one field period, strictly speaking 262 H periods).
Reference numeral 8 designates a change-over switch which is changed in position at every 1/2 H period. Accordingly, the video signal of 1 H period amount read out from the 1 H memory 2A or 3A and the video signal of 1 H period amount read out from the 1 H memory 2B or 3B are sequentially supplied through this change-over switch 8 to an output terminal 6 at every 1/2 H periods.
In the example shown in FIG. 2, at the output terminal 6 appears a video signal S.sub.NI ' (shown in FIG. 4C) of non-interlace system which has the horizontal frequency doubled and in which the video signal on each scanning line of the present field and the corresponding video signal on the scanning line of the preceding field appear alternately at every 1/2 H periods.
FIG. 3 shows a method of predicting that a video signal on the scanning line to be interpolated equals the arithmetic average of the video signals on the preceding and succeeding scanning lines and employing the above video signal as an interpolated signal, namely, a so-called arithmetic average-field interpolation method.
In FIG. 3, reference numerals 2c and 3c respectively designate 1 H memories and 4c and 5c change-over switches which operate in the same way as the 1 H memories 2 and 3 and the change-over switches 4 and 5 shown in FIG. 1. Accordingly, through the change-over switch 5C there is derived a video signal in which the video signal of the input video signal S.sub.i on each scanning line appears twice continuously at every 1/2 H periods. This video signal is supplied through a delay line 9 having a delay amount of 1/2 H period to an adder 10. Also, this video signal is directly supplied to the adder 10 so that the adder 10 generates the output signal resulting from adding two video signals, which is then adjusted to be the 1/2 level by a level adjuster 11 and then fed to the output terminal 6.
Thus, according to the example shown in FIG. 3, at the output terminal 6 appears a video signal S.sub.NI " (shown in FIG. 4D) of non-interlace system which has the horizontal frequency twice that of the original video signal and in which a video signal of the input video signal S.sub.i on each scanning line and a video signal resulting from arithmetically averaging two video signals, namely, this video signal and a video signal on the succeeding scanning line alternately appear at every 1/2 H periods.
If the video signals S.sub.NI to S.sub.NI " of a non-interlace system, each having the horizontal frequency twice that of the original video signal provided as described above, are supplied to a picture tube so as to perform the double-speed scanning, the field flicker and line flicker as described above will never occur.
However, a double scanning television receiver which employs the above video signals S.sub.NI to S.sub.NI " of a non-interlace system to perform the double speed scanning has the following defects.
In the double-scanning television receiver which employs the video signal S.sub.NI of a non-interlace system having the interpolated signal provided according to the pre-value interpolation method as shown in FIG. 1, the scanning line of the same video signal succeeds twice so that a line in the oblique direction becomes stairs or zig-zag. Although this stairs or zig-zag-shape line is inconspicuous in the still picture, this causes the significant deterioration of picture quality in the moving picture. This defect becomes more serious as the size of picture screen becomes larger.
In the double-scanning television receiver which employs the video signal S.sub.NI ' of a non-interlace system having the interpolated signal provided according to the pre-field interpolation method as shown in FIG. 2, the scanning line of the video signal on the present field and the scanning line of the video signal on the preceding field are alternately displayed on the picture screen so that this television receiver is ideally suitable for the picture having a strong correlation in the direction of time, namely, the still picture. However, in the moving picture, time difference occurs within the picture and hence the picture quality is deteriorated.
Further, in the double-scanning television receiver which employs the video signal S.sub.NI " of a non-interlace system having the interpolated signal provided according to the pre-value average interpolation method as shown in FIG. 3, since the video signal on the scanning line interpolated equals to the arithmetic average of the video signals on the preceding and succeeding scanning lines, the integral action thereof causes the resolution in the vertical direction to be deteriorated. However, in this case, the fact that the line in, for example, the oblique direction becomes stairs is alleviated as compared with the case of the example shown in FIG. 1.
As described above, according to the pre-value interpolation method shown in FIG. 1, the line in, for example, the oblique direction becomes stairs, which then causes the picture quality to be deteriorated. On the other hand, according to the pre-value average interpolation method shown in FIG. 3, although the integral effect alleviates the deterioration of the above picture quality, this method is substantially the same as connecting a low-pass filter in the vertical direction, thus the resolution being lowered.
Consider a window pattern as, for example, shown in FIG. 5. According to the pre-value average interpolation method as shown in FIG. 3, the resolution of the sides AB and CD is deteriorated and hence a so-called "blur" occurs.
By the way, the video signal region in which the picture quality is deteriorated according to the pre-value interpolation method shown in FIG. 1 is the region or area having high frequency band components in the vertical and horizontal directions like the line in the oblique direction in FIG. 5. The area having the high frequency band components is such an area in which the brightness changes abruptly. In the window pattern shown in FIG. 5, only the corners A, B, C and D correspond to such area.